This invention relates to a digital video signal reproducing apparatus, and more particularly to an apparatus of the kind described above in which compression and expansion of picture data recorded on a magnetic tape can be carried out at the time of reproduction without substantially degrading the picture quality as well as the sound quality.
Digital magnetic recording and reproducing apparatuses recording and reproducing a video signal after converting it into a digital signal have been developed and put into practical use. As an example of these apparatuses, there is an apparatus which is based on a method of recording as described in a document entitled "D2 NTSC Composite Digital VTR", Technical Report of the Institute of Television Engineers of Japan, 11, 24, pp. 13-18 (1987). This technical report is based on the same ground as that defined in the D2 Standards of SMPTE (Society of Motion Picture and Television Engineers) of U.S.A.
The recording format of this digital magnetic recording and reproducing apparatus is shown in FIG. 13. Referring to FIG. 13, digital video data of one channel and digital audio data of four channels are recorded on helical tracks HLT on a magnetic tape. A CTL signal, a time code TC and a CUE signal are recorded on a control track CTT, a time code track TCT and an analog audio track AOT on the magnetic tape, respectively. This manner of data recording is attained, for example, by the use of a drum 1 as shown in FIG. 14. Referring to FIG. 14, the drum 1 includes four recording heads REC1 to REC4 used for recording data of two channels, and one field is recorded on the magnetic tape by 1.5 rotations of the cylinder. The recorded data is reproduced by four playback heads PB1 to PB4 provided for the exclusive purpose of reproduction.
JP-A-59-89085 discloses such a digital video signal recording and reproducing apparatus which can carry out compression and expansion of picture data, recorded on a magnetic tape, at the time of reproduction without substantially degrading the picture quality as well as the sound quality. For the purpose of compression and expansion of recorded data at the time of reproduction, the magnetic tape is driven at a non-standard speed, and the drum 1 is also rotated at the non-standard speed, thereby temporarily storing all the data recorded on the magnetic tape in a memory, so that the data belonging to, for example, one field can be dropped out or inserted.
However, the prior art apparatus described above, in which both the magnetic tape and the drum are driven at the non-standard speed in the playback mode, has been defective in that the frequency of the recorded data reproduced from the magnetic tape tends to change, resulting in an increased error rate of the recorded data reproduced from the magnetic tape, thereby greatly degrading the picture quality as well as the sound quality at the time of reproduction.
The prior art apparatus has also been defective in that, when the speed of reproduction is abruptly changed, the magnetic heads tend to run out from the tracks, resulting similarly in undesirable degradation of both the picture quality and the sound quality.
Further, the prior art apparatus has had such another problem that, when a conventional pitch controller available on the market is used for the reproduction of the tone signal, the tone pitch cannot be accurately corrected, even when the speed of reproduction is detected on the basis of the time code TC so as to correct the tone pitch. The conventional pitch controller requires suitable means for correcting the tone pitch, because the reproduced tone pitch changes with the change in the moving speed of the magnetic tape.